1. Field of the Invention
The present invention relates to display devices and methods for controlling the same, and in particular, to a method for detecting a variation in characteristics of semiconductor driving active elements.
2. Description of the Related Art
Image display devices in which organic EL elements (also known as organic light emitting diodes, or OLEDs) are used, that is, organic EL displays are known as image display devices with which current-driven luminescence elements are used. Organic EL displays are attracting attention as candidates of the next-generation flat panel display (FPD) because they have advantages of good viewing angle properties and small power consumption.
In a usual organic EL display, organic EL elements which serve as pixels are arranged in a matrix. An organic EL display is called a passive-matrix organic EL display, in which organic electroluminescence elements are provided at intersections of row electrodes (scanning lines) and column electrodes (data lines) and voltages corresponding to data signals are applied to between selected row electrodes and the column electrodes to drive the organic EL elements.
On the other hand, an organic EL display is called an active-matrix organic EL display, in which thin film transistors (TFTs) are provided at intersections of row electrodes (scanning lines) and column electrodes (data lines) and connected with gates of driving transistors which receive data signals, when the TFTs are turned on through selected scanning lines, through the data lines and activate the organic EL elements.
Unlike the passive-matrix organic EL display, in which organic EL elements connected to selected row electrodes (scanning lines) emit light only until the selected row electrodes become unselected, organic EL elements in the active-matrix organic EL display keep emitting light until they are scanned (or selected) again; thus causing no reduction in luminance even when a duty ratio increases. Accordingly, the active-matrix organic EL display is operated at a low voltage, thereby consuming less power. However, a problem of unevenness in luminance occurs in the active-matrix organic EL display because luminances are different among pixels due to a variation in characteristics of driving transistors or organic EL elements even when the same data signals are provided.
In conventional organic EL displays, such unevenness in luminance due to a variation or degradation in characteristics (hereinafter collectively referred to as unevenness in characteristics) of driving transistors or organic EL elements has typically been compensated by using complicated pixel circuitry or by feedback compensation using a representative pixel or the sum of currents flowing in all the pixels.
Using complicated pixel circuitry, however, reduces yields. Feedback compensation using a representative pixel or the sum of currents flowing in all the pixels cannot compensate unevenness in characteristics among pixels.
For these reasons, several methods have been proposed for detecting unevenness in characteristics among pixels using simple circuitry.
For example, for a substrate for a luminescent panel, a method for testing the substrate for the luminescent panel, and a luminescent panel disclosed in Patent Reference 1 (Japanese Unexamined Patent Application Publication Number 2006-139079), pixels are tested and characteristics of the pixels are extracted by detecting relationship between a data voltage and a current flowing in a driving transistor by measuring, before the EL element is formed on the substrate for a luminescent panel, a current flowing in a test line connected to a diode-connected transistor which is connected to a conventional voltage-driven pixel circuit including two transistors and serves to resemble an EL element. After the EL element is formed, the diode-connected transistor can be made reverse-biased using the test line, so that a current is prevented from flowing in the diode-connected transistor and thereby usual operation of writing a voltage can be performed. The characteristics detected as data items of a matrix can be utilized for controlling correction of voltage applied to a data line when an organic EL panel is used.
However, a drive current flowing in pixels is so fine that it is difficult to accurately measure such a fine current via a line, such as a test line, for measuring the current.
For the substrate for a luminescent panel, the method for testing the substrate for the luminescent panel, and the luminescent panel disclosed in the Patent Reference 1, accuracy in detection of characteristics of the driving transistor is poor because the characteristics are detected by measuring current. As a result, accuracy in detection of a variation in characteristics of driving transistors is so poor that unevenness among pixels is not corrected sufficiently.
The driving transistors of the pixels are connected to a common power supply and a common electrode in the luminescent panel. The test line described in the Patent Reference 1 is also connected to the common power supply and the common electrode in the light-emitting diode. Measurement of a fine current with good accuracy is difficult because the driving transistors are connected to the common electrode and the common power supply and thus the measurement is subject to influence of noise caused by a component other than a pixel which is currently being measured or influence of voltage drop or change in impedance due to load status of a component other than a pixel which is currently being measured.
Furthermore, as typified by the detection of the variation in characteristics of the driving transistors through the measurement of a fine current described in the Patent Reference 1, such a detection operation needs to be performed in an additionally provided period in which the luminescent panel actually does not perform a display operation. The period in which a display operation is performed may be limited because of the detection operation in the case, for example, where it is necessary that a variation in characteristics of the driving transistor is periodically detected to correct change with time.
The present invention, conceived to address the problem, has an object of providing a display device which allows, even with simple pixel circuitry, highly efficient and accurate detection of current of a driving active element of each pixel and a method for controlling the display device. The present invention also has an object of providing a method for detecting a variation in characteristics of the driving active element of each pixel with high accuracy using a result of the detection of the current.